This invention relates generally to security passageways, and particularly to security doors and the sensing of unauthorized passage of objects as well as people through the doors.
Security doors are used in airports, banks, commercial buildings, military installations, and other locations where restricted access is desirable. One type of security door is a revolving door with a plurality of passenger compartments defined by panels. For example, such a revolving door is disclosed in U.S. Pat. No. 4,627,193 issued on Dec. 6, 1986. Normally, in this type of door a person inserts a pass card into a card identifying device linked with a control mechanism for the door, then enters a compartment on one side of the door. If the card is authorized, the door will turn its panels and thus each compartment until the entered compartment moves from the entrance to the exit. As the entered compartment passes from the entrance to the exit, all of the other compartments move by a corresponding amount. Therefore, it is possible for an unauthorized person to "tailgate", i.e., to either enter the compartment immediately following the authorized party as it passes the entrance, or enter a compartment located at the exit. In the former situation, the unauthorized party will get trapped in the doorway when the door stops. If the door has a "trapped man" feature to detect such a situation, the system will reversely rotate the door after stopping to force the unauthorized "trapped" person back to his starting point. If the door is not equipped with such a "trapped man" feature, the next authorized party to enter the doorway will allow the unauthorized party to pass to the exit. In the situation where the "tailgater" is attempting to pass from the exit to the entrance, the system may also detect him and return him to his starting point before allowing his compartment to reach the entrance.
One way that tailgating has been detected is by the use of floor mats in the security door to detect when a compartment has been entered. However, such mats have several drawbacks. First, rain, snow, dirt or other foreign matter can often cause mat failures. Second, a mat cannot detect a person or object such as a gun or a security pass card attached to the door frame. Third, it is difficult to make a mat sufficiently sensitive to lightweight objects. Accordingly, there is a need to more accurately and reliably detect whether unauthorized parties or objects have entered a compartment of a revolving security door.
It has been proposed to use ultrasonic sensors instead of mats, but use of such sensors in a revolving door presents difficult problems. First, the sensors must distinguish between door panels and people or objects. If the sensors are merely turned off when the door panel passes by, objects attached to or close to the door panel can get through the doorway undetected. Second, to detect small objects such as pass cards or firearms, the sensors must have a high gain. Such a high gain increases the likelihood that reverberations or echoes will cause false readings. This is especially true in a security revolving door which has a substantially closed housing. Similarly, the greater the range (portion of the floor to ceiling distance) covered by the sensor, the greater the likelihood of false readings due primarily to echoes from the floor. Accordingly, floor mats have been popular in security door devices.